1319
Znd. Eng. Chem. Res. 1991,30, 1319-1323 Allcock, H. R.; Levin, M. L.; Austin, P. E. Quaternized Cyclic and High Polymeric Phoephazenes. Inorg. Chem. 1986,25,2281-2288. Allcock, H. R.;Connelly, M. 5.; Sisko, J. T.; Al-Shali, S. Effects of Organic Side Group Structures on the Properties of Poly(organophosphazenes). Macromolecules 1988,21,323-334. Austin, P. E.; Riding, G. H.; Allcock, H. R. Improved Method for the Synthesis of Poly(organophosphazenes) and Hindered Cyclophosphazenes. Macromolecules 1983,16,719-722. Brandt, K. Synthesis and Properties of the New Reactive Oligomers Containing CyclophosphazeneMoieties. Chem. Stosow. 1986,30, 255-271. Brandt, K. Novel Segmental Polymers Containing Alternating Rigid and Soft Blocks. Acta Polym. 1988,39(1/2),13-17. Brandt, K. Syntheses and Structures of Precursors in the Polycondensation of Hexachlorocyclotriphosphazeneand Hydroquinone. Inorg. Chim. Acta 1989,157,251-258. Carr, L.J.; Nichols, G. M. Process for Preparation of Phosphazene Esters. U.S.Patent 4600791,1986,p 4. Dehmlow, E. V.; Dehmlow, S. S. Practical Applications of Phase Transfer Catalysis. Phase Transfer Catalysis, 2nd ed.; Verlag Chemie: Weinheim, 1983;pp 52-321. Dell, D.; Fitzsimmons, B. W.; Shaw, R. A. Phosphorus-Nitrogen Compounds. Part XIII. Phenoxy- and p-Bromophenoxy-chlorocyclotriphosphazenes. J. Chem. SOC.London 1965,4070-4073. Kumar, D.; Fohlen, G. M.; Parker, J. A. High-Temperature Resins Based on Aromatic Amine-Terminated Bisaspartimides. J. Polym. Sci., Polym. Chem. Ed. 1983a,21 (l), 245. Kumar, D.; Fohlen, G. M.; Parker, J. A. Bis-, Tris-, and Tetrakis-
maleimide PhenoxytriphenoxycyclotriphosphmeneResins for Fire- and Heat-resistant Applications. Ibid. 1983b, 21 (ll), 3155-3167. Kumar, D.; Fohlen, G. M.; Parker, J. A. High-Strength Fire- and Heat-Resistant Imide Resins Containing Cyclotriphoephezene and Hexafluoroisopropylidene Groups. Ibid. 1984,22 (4),927-943. Kumar, D.; Fohlen, G. M.; Parker, J. A. Polybismaleimide Containing Tetrakisphenoxycyclotriphosphazenes. Ibid. 1985, 23 (6), 1661-1670. Kumar, D.; Fohlen, G. M.; Parker, J. A. The Curing of Epoxy Resins with Aminophenoxycyclotriphosphazenes. Ibid. 1986,24 (lo), 2415-2424. McBee, B. T.; Okuhara, K.; Morton, C. J. Reaction of Hexachloroand 2,2,4,4-Tetrachloro-6,6-diphenylcyclotriphosp~trienes with Sodium Phenoxide. Inorg. Chem. 1966,5,450-457. Morgan, P. W. Comments on the Status and Future of Interfacial Polycondensation. J. Macromol. Sci., Chem. 1981, A15 (5), 683-699. Reuben, J. Carbon-13 and Phosphorus-31 NMR Spectroscopy of Phenoxychlorocyclotriphosphazenes,N,P3C&-,,(OC6H6),. Magn. Reson. Chem. 1987,25, 1049-1053. Wang, M. R.; Wu, H. S. Effects of Mass Transfer and Extraction of Quaternary Salts on a Substitution Reaction by Phase-Transfer Catalysis. J. Org. Chem. 1990,55,2344-2350.
Received for review July 26, 1990 Accepted December 26,1990
Synthesis of Barium Ferrite Ultrafine Particles by a Hydrothermal Method Eizo Sada,* Hidehiro Kumazawa, and Hae-Man Cho Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan
Barium ferrite ultrafine particles were synthesized hydrothermally from aqueous mixed solutions of ferric nitrate and barium nitrate. The effect of the reaction conditions on the size of hexagonal platelike particles was investigated. As the hydrothermal temperature rises, the particle size increases. This may be because at higher temperatures and higher pressures, the particle growth rate increases. The mean size of the particles decreases with increasing stirring speed. As the alkali molar ratio defined by [OH-]/ [NO3-] increases, the mean size of the particles decreases. Ultrafine particles of less than 0.1 pm, which are suitable for perpendicular magnetic recording media, can be synthesized by hydrothermal treatment under the conditions of temperature of 300 "C,alkali molar ratio of 6, and stirring speed of 300 rpm.
Introduction For the production of functional solid materials, control of particle shape and size is essential. Barium ferrite ultrafine particles of hexagonal plate shape, which are suitable for perpendicular magnetic recording media, are a typical example. Perpendicular magnetic recording mode is suitable for high-density recording. The particles for this purpoee must satisfy the following requirements: they are of hexagonal plate shape with a mean size of 0.1 pm or less and the size distribution is sharp. Until now, the shape and size control was achieved because of vast experimental data. The synthesis of barium ferrite particles by a hydrothermal method has actually been reported by several researchers (Kiyama, 1976; Barb et al., 1986; Yoshimura et al., 1989), but the relationship between particle size and operating conditions has not been investigated systematically. To our knowledge, the operating conditions for production of ultratine particles of less than 0.1 wm in size have not yet been established. If the reaction scheme leading to formation of the ultrafine particles and
*Towhom correspondence should be addressed.
the relationship between the shape and size of the particles and the reaction rate can be clarified, then the conditions for producing particles with the desired shape and size may be determined with high reliability and reproducibility. In the present research, ultrafine particles of barium ferrite were synthesized from a mixture of ferric nitrate and barium nitrate as raw materials by using a hydrothermal method. The relation of the particle shape and size to such operating conditions as alkali (NaOH) concentration, stirring speed, hydrothermal temperature, and reaction time were investigated systematically. The operating conditions to produce ultrafine particles of thin hexagonal plate shape of size less than 0.1 pm were developed, and a mechanism of particle growth was proposed.
Experimental Section A stainless steel autoclave (Nitto, UN-4) equipped with a stirrer (Nitto, NS-8) was used as a reactor operated under hydrothermal conditions. Barium ferrite ultrafine particles were synthesized from aqueous mixed solutions of ferric nitrate and barium nitrate. The procedure for the synthesis was as follows: first, an aqueous solution contsi??ing ferric nitrate and barium nitrate with a molar ratio of Fe
0888-5885/91/2630-1319$02.50/00 1991 American Chemical Society
1320 Ind. Eng. Chem. Res., Vol. 30,No. 6, 1991
Afterward, the particles were observed by means of a scanning electron micrograph (SEM,Hitachi, S-510). Further, some samples were examined by X-ray diffraction (Shimazu, XD-610).
Experimental Results and Discussion Addition of sodium hydroxide to an aqueous solution of ferric nitrate with an excess stoichiometry yields ultrafiine particles of ferric hydroxide whose size ranges from 3 to 4 nm (van der Gissen, 1966). Hydrothermal treatment of such a suspension of ferric hydroxide at 300 "C for 5 h formed fine particles of platelike shape. Figure 1shows a scanning electron micrograph (SEM) of produced particles. The particles are found to be 1-2 pm in size and of thin platelike shape. From X-ray diffraction measurements depicted in Figure 2 the particles were proven to be crystalline a-Fe203. It is, therefore, considered that barium ferrite is formed when Ba(OH)2 coexists in an appropriate proportion under hydrothermal conditions 90 that Fe(OH)3may be converted to Fe20s The reaction can be written stoichiometrically as 12Fe(OH)3+ Ba(OH)2 BaO-6Fe203+ 19H20 (1)
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Figure 1. Scanning electron micrograph of a-Fe203particles.
to Ba fixed at 8 was prepared. Subsequently, an aqueous NaOH solution with a prescribed concentrationwas added, and the resultant slurry was agitated for about 30 min. A 50-cm3portion of the slurry was fed into a pot made of titan and was put into the autoclave. The rate of rise-intemperature to a specified hydrothermal temperature was 4 "C/min. The stirring speed under the hydrothermal condition was varied to 500 rpm. The hydrothermal temperature ranged from 200 to 300 "C. The hydrothermal treatment was continued for 20 h. Prepared ultrafine particles were rinsed with distilled water several times and dried at about 100 O C in an oven.
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Figure 6. Scanning electron micrograph of barium femte particles prepared under 300 O C , alkali molar ratio of 6, and stirring speed of 300 rpm for 5 h.
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pared from dispersion of barium ferrite powders with coexisting aerosol OT (AOT), whereas the lower photo shows the sample prepared in the absence of AOT. From the upper photo, barium ferrite powders are found to be of hexagonal platelike shape. From the lower photo, the hexagonal platelike particle is magnetized in the direction vertical to the hexagonal plate plane. The mean size and thickness of prepared particles were determined to be 0.67 and ca. 0.1 pm from these photos. The influence of stirring speed on the size of particles was investigated when prepared under a constant temperature of 300 "C for 5 h. The relationship between the
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mean size and the stirring speed was plotted for two levels of alkali molar ration in Figure 5. It is apparent that the mean size of particles decreases with increasing stirring speed. At the molar ratio of 6, the mean size becomes less than 0.1 pm when the stirring speed exceeds 300 rpm. Figure 6 demonstrates a typical example of SEM images of particles whose size is less than 0.1 pm. It is judged from Figure 3 that both nucleation and crystal growth rates increase with an increase in stirring speed, but the nucleation rate overwhelms the crystal growth rate. The effect of alkali molar ratio on the size of particles was examined under constant speed of 300 rpm, hydrothermal temperature of 300 "C, and treatment time of 5 h. The relationship between mean size of particles and alkali molar ratio is shown in Figure 7. As the alkali molar ratio increases, the mean particle size decreases. Particles less than 0.1 p m are produced when the alkali molar ratio exceeds 6. The ratio of thickness to size of particles (i.e., aspect ratio) varied from 3 to 5. The aspect ratio increases with increasing size. Figure 8 illustrates the X-ray diffraction patterns of particles prepared under molar ratios of 3,6, and 8. The particles prepared at molar ratios of 3 and 6 are proven to be crystal barium ferrite. However, strengths of some peaks in the diffraction pattern of particles prepared under the molar ratio of 8 are reversed as compared with those of standard barium ferrite particles. Hence it was judged that some different crystals of very small sizes might also be formed or some different crystalline phase of barium ferrite might appear. Thus, for the preparation of crystalline barium ferrite particles, it is safe to keep the molar ratio less than 6.
1322 Ind. Eng. Chem. Res., Vol. 30, No. 6,1991
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The effect of hydrothermal temperature on the particle size was investigated under constant stirring speed and treatment time. The variations of size of prepared particles with temperature for two levels of alkali molar ratio are shown in Figure 9. When the alkali molar ratio was fixed a t 3, the mean particle size increased with increasing hydrothermal temperature. From such experimental evidence, particles grow larger at higher temperatures or pressures, because the particle growth rate goes up, the term in which both particle growth and nucleation overlap each other becomes shorter, and accordingly the number of host nuclei is reduced. The powders synthesized at temperature below 225 "C, however, have only low crystallinity and are not suitable for magnetic materials. Also, when the molar ratio is fixed at 6, powders formed at temperatures below 250 "C are not suitable for magnetic
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materials because of low crystallinity. Though ultrafine barium ferrite particles of less than 0.1 pm in size can be synthesized at temperatures above 275 "C,in view of crystallinity of the particles the hydrothermal temperature of 300 "C is needed. Thus far, the hydrothermal treatment time was fixed at a constant time of 5 h. In the following, the variation of particle size with treatment time was investigated. Figure 10 indicates the time dependence of the mean size of particles prepared under constant hydrothermal temperature (300 "C)and stirring speed (300rpm). When the treatment time exceeds 1h, particles grow slowly. Though in the early stage of hydrothermal treatment, nucleation and particle growth coexist, it is judged from such experimental evidence that the particle growth by Ostwald ripening dominated after about 1 h.
1323
Ind. Eng Chem. Res. 1991,30,1323-1329 300 rpm 0.4
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Figure 9. Relationships between mean particle size and hydrothermal temperature for two levels of alkali molar ratio.
creases, the term in which both nucleation and particle growth overlap each other becomes shorter, and as a result the number of host nuclei is reduced. The mean size of particles decreases with increasing stirring speed. This experimental evidence may be ascribed from the fact that the nucleation rate rises in the early stage of the reaction and hence numerous host nuclei are generated. To prepare crystal barium ferrite, the hydrothermal temperature must go up to about 300 "C. As the alkali molar ratio increases, the mean size of particles decreases. Ultrafine particles of less than 0.1 pm can be synthesized by the hydrothermal treatment under the conditions of an alkali molar ratio of 6, temperature of 300 "C, and stirring speed of 300 rpm.
Nomenclature d = mean size of hexagonal platelike particles, pm R.3
n = stirring speed, rpm R = alkali molar ratio defined by [OH-]/[NO,-] t = thickness of hexagonal platelike particles Registry No. Barium ferrite, 12047-11-9;iron nitrate, 10421-48-4;barium nitrate, 10022-31-8.
Literature Cited Time, h Figure 10. Variation of mean particle size with time for two levels of alkali molar ratio.
Conclusion Barium ferrite ultrafine particles were synthesized hydrothermally from aqueous mixed solutions of femc nitrate and barium nitrate. The following findings were yielded After 1-2 h of hydrothermal treatment, the particle growth driven by Ostwald ripening dominated. At higher temperatures, that is, higher pressures, particles grow larger, because the particle growth rate in-
Barb, D.; Diamandescue, L.; Ruci, A.; Mihaila, D. T.; Moraeiu, M.; Teodorescu, V. Preparation of Barium Hexaferrite by a Hydrothermal Method: Structure and Magnetic Properties. J . Mater. Sci. 1986,21, 1118. Kiyama, M.Conditions for the Formation of Compounds Consisting of BaO and FezOafrom Aqueous Suspension. Bull. Chem. SOC. Jpn. 1976,49, 1855. van der Gissen, A. A. The Structure of Iron(II1) Oxide-Hydrate Gels. J . Inorg. Nucl. Chem. 1966,28, 2155. Yoshimura, M.; Kubodera, N.; Noma, T.; Somiya, S. Synthesis of Ba-Ferrite Fine Particles by Hydrothermal Attrition Mixing. J . Ceram. SOC.Jpn. 1989, 97,16.
Received for review July 3, 1990 Accepted January 14,1991
Measurements of Binary Diffusion Coefficients of C16-C24Unsaturated Fatty Acid Methyl Esters in Supercritical Carbon Dioxide Toshitaka Funazukuri, Sumito Hachisu, and Noriaki Wakao* Department of Chemical Engineering, Yokohama National University, Yokohama 240, Japan
Binary diffusion coefficients were measured for Cls-Czr unsaturated fatty acid methyl esters in supercritical carbon dioxide (at 313 K and 16.0 MPa) and linoleic acid methyl ester in supercritical carbon dioxide (at 308-328 K and 14.0-33.6m a ) . We propose an empirical equation that correlates the diffusion coefficients in terms of Schmidt numbers, (Sc - Sc*)/Sc*, with molar volumes, (u2 ( U ~ ) ~ > / ( L Jwhere ~ ) ~ , Sc and Sc* are Schmidt numbers at the prescribed pressure and a t atmospheric pressure, respectively, and where u2 and ( u ~are ) ~molar volumes a t the prescribed pressure and a t which viscous flow ceases, respectively. Also, it was found that C02self-diffusion coefficient values recommended by Chen (1983) were well correlated by this equation.
